1. Field of the Invention
The present invention relates to a method and an apparatus for carrying out the annealing step of a galvannealing process in which strips and sheets, particularly of steel, are subjected after galvanizing to an annealing step by heating the coated material and subsequently holding at final annealing temperature.
2. Description of the Related Art
When hot-galvanized sheet or strip steel is annealed after dipping at temperatures above the melting point of zinc, the resulting product is called galvannealed sheet or strip and the process is called galvannealing, i.e., by combining the expressions xe2x80x9cgalvanizingxe2x80x9d and xe2x80x9cannealingxe2x80x9d.
The coating of the coated strip treated in this manner is composed only of iron/zinc compounds with about 10-12% Fe.
A conventional hot galvanizing process is carried out before the annealing step of a galvannealing process. For this purpose, the steel surface is cleaned first. Subsequently, a recrystallizing annealing of the initial material which is hot from rolling is carried out in a furnace in a protective gas atmosphere. The strip is then cooled to galvanizing temperature and is hot galvanized by means of an aluminum-containing zinc melt. Finally, the excessive zinc melt is stripped off by means of air or nitrogen.
In a steel strip whose surface has been coated in this manner the galvannealing process is completed by a subsequent annealing step in an additional furnace.
During annealing, a diffusion-controlled process takes place between the steel matrix and the zinc coating. In dependence on the temperature adjusted during annealing and the annealing time, different FeZn phases in accordance with the zinc/iron phase diagram are formed. The respective phase components determine the total iron content of the coating.
The phase composition adjusted during this annealing step is a decisive factor for the quality of the coating and the usefulness of the basic material treated by galvannealing, for example, for the later deep-drawing process in the forging press.
In conventional plants this galvannealing furnace is composed of two zones: first, the zone for inductively heating the strip and, second, the subsequent zone for holding at the desired final temperature. The holding zone is usually heated by resistance-heated or gas-fired furnace portions.
The annealing step of the galvannealing process and, thus, obtaining a defined base composition of the coating material are dependent in particular on the parameters temperature and time. These important parameters can be influenced by the plant parameters, the strip entry temperature into the zinc melt, the temperature of the zinc melt, the aluminum concentration in the zinc melt as well as the thickness of the coating. The most important variable is the basic material, i.e. the alloy composition of the steel and its condition.
Fine sheet metal treated by galvannealing is used predominantly in the automobile industry and is distinguished by a good weldability and by the fact that it can be easily varnished.
In the past, IF-steels (interstitial-free steels) have been used almost exclusively in this field as a base material for deep-drawn products, special-deep drawn products and extra-deep drawn products for a galvannealing treatment.
IF-steels are those steels which in the iron lattice do not have any interstitially dissolved atoms. The C-atoms and the N-atoms are bound by specifically adding carbonitride formers (Ti, Nb, V) to the alloy. IF-steels have no significant contents of strength-increasing elements such as P, Mn or B. On the other hand, the element Si can be added to the alloy (up to about 0.10%) for improving the adherence of the galvannealed coating.
In order to meet the requirement for a weight reduction of automobiles, increasingly thinner sheet steels are used which, however, must have the same strength properties as conventional sheet steels. This requirement can only be met by using higher-strength steels and also higher-strength IF-steels. Higher-strength IF-steels have significant portions of the above-mentioned elements. When higher-strength steels are mentioned in the following, they are intended to also include higher-strength IF-steels, BH-steels and TRIP-steels.
However, the two steel groups IF-steels and higher-strength steels have in relation to the applied zinc coating a significantly different alloying behavior, particularly with respect to their speed. The alloy formation takes place in the higher-strength steels substantially slower than in the IF-steels.
Therefore, it is the primary object of the present invention to propose a method and an apparatus for the annealing step of a galvannealing process to which sheets and steels of different base materials, particularly of higher-strength steels can be subjected without negatively affecting the output.
In accordance with the present invention, during annealing, the heating step is interrupted by at least one additional holding step and, thus, a stepwise increase of the temperature over time is adjusted.
The apparatus according to the present invention for carrying out the annealing step of a galvannealing process includes a zone for inductively heating the strip and an additional zone for holding the strip at final temperature, wherein the zone for inductively heating the strip is interrupted by at least one additional holding zone.
The basic concept of the present invention is the adjustment of the annealing cycle with respect to the parameters temperature and time to the base materials, particularly higher-strength steels, for taking into consideration the material-specific alloying progress. The proposed process technology including the stepwise annealing treatment provides the possibility of a controlled adjustment of the properties between the base material and the coating material and the coating material itself.
This annealing treatment is advantageously carried out in such a way that the heating process with subsequent holding at final temperature is interrupted by a second holding process at a temperature below the final temperature.
A furnace suitable for the apparatus according to the present invention includes a zone for inductively heating the strip and an additional zone for holding the strip at the heating temperature, wherein at least one additional holding zone is provided between the zone for inductively heating the strip and the final holding zone.
In accordance with an advantageous embodiment which is particularly suitable for higher-strength steel strips, this furnace is composed of four zones, namely, a first induction zone with a subsequent first holding zone, and a second subsequent induction zone with a subsequent second holding zone.
The proposed method for the annealing treatment carried out step-by-step or in stages in a galvannealing process and the proposed configuration of the annealing furnace have the following advantages.
By carrying out the increase of the temperature in a stepwise manner, an adjustment of the annealing treatment to the slower diffusion processes and, thus, alloying speeds in higher-strength steels is achieved. The alloying process can be controlled and regulated. This makes possible a uniform product quality under controllable production conditions. This stepwise heating process does not exhibit any disadvantages in IF-steels.
The annealing parameters, particularly the heating temperature and heating speed, are adapted to the alloying sequence of the combination steel/coating material. Consequently, overheating in the coating material does not take place without an alloy formation. Moreover, the possibility of an increased evaporation of zinc is reduced. This constitutes a significant advantage for the operation of the galvannealing furnace as well as for the morphology of the galvannealed coating.
For avoiding an overheating of the zinc coating in conventional galvannealing furnaces, which have only one single heating zone composed of several or only one single induction coil, and for adjusting a controlled alloying process, the capacity of the induction zone would have to be lowered. In order to still be able to reach the desired galvannealing temperature, this would make it necessary to reduce the speed of the plant. However, this would result in a reduction of the output of the hot galvanizing plant.
In contrast, the method and apparatus proposed in accordance with the present invention do not result in a decrease of the output of the hot galvanizing plant.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.